In the atmosphere around cities and industrial plants, there are many small particles of matter.
An aerosol may be defined as a group of solid particles or liquid particles suspended in a gaseous medium. The size range of these particles is generally between 10 nanometers and 100,000 nanometers in diameter. In an aerosol, the large particles account for most of the mass or weight of the aerosol. From observation, it appears that the particle sizes between 100 nanometers and 1000 nanometers cause the greatest health impairment and also cause the greatest decrease of visibility in the atmosphere.
Prior to this invention, there was no convenient means or method to measure the size distribution for particles in the size range of 10 nanometers to 300 nanometers in the atmosphere. If the size of the particles in an atmosphere could be measured to 10 nanometers, it is reasonable to conclude that means and methods could be found to control these fine particles in the atmosphere and to remove these fine particles so as to lessen the danger to the health.
Examples of pollutants and particulate matter in the atmosphere are the effluent from a plant burning coal, effluent from an aluminum reduction plant, and the general particulate matter in the atmosphere.
The effluent from a plant for burning coal comprises particulate matter ranging in size, as determined by a cascade impactor prior to this invention, of particles having a diameter in the range of 300 nanometers to 10,000 nanometers. The cascade impactors prior to this invention were not capable of measuring the particle size to a diameter less than 300 nanometers. The effluent from a coal burning plant comprises a, relatively, wide reange of particle sizes. The larger particles of the atmosphere, relatively, close to the coal burning plant while the smaller particles will settle out of the atmosphere at a greater distance from a coal burning plant. And, the smallest particles in the effluent will not settle out from the atmosphere but will be washed out of the atmosphere by rain and snow and the like. It is my understanding that, the particle sizes between 50 nanometers and 1000 nanometers pose the greatest problems to health. For example, at the present time, it is believed that the particles having a diameter in the range of 50 nanometers to 1000 nanometers pose the greatest health hazard and the particles having a diameter in the range of 100 nanometers pose the greatest bisibility problems. The particulate matter in the effluent from a coal burning plant poses problems with respect to determining the size of the particulate matter and also in removing the particulate matter from the effluent. At the present time, one of the biggest problems is the determination of the size of the particulate matter in the effluent.
Another example is the size of the particulate matter in the effluent from an aluminum reduction plant. As is well known, in an aluminum reduction plant there are used electrodes. The electrodes are made from a paste of carbon particles in a hydrocarbon matrix. For example, the anode may be formed in the Soderberg process by continually adding paste and letting the hydrocarbon bake or heat and cook to form an anode. In the formation of the anode, there is given off a large amount of hydrocarbons. Or, the anode may be formed in a separate facility so as to be a prebaked anode and then inserted into the potline for making the molten aluminum. In the facility for prebaking the anode, there is also given off a large amount of hydrocarbon. The hydrocarbons are given off into the atmosphere and, because of a nucleation process taking place in the atmosphere, are condensed to form a haze, such as a typical blue haze. At the aluminum reduction plant at Tacoma, Washington, the effluent from the plant was measured by a cascade impactor, prior to the cascade impactor of this invention, and the particle size ranged from a diameter of 300 nanometers to 10,000 nanometers. There is scientific reason to believe that in the effluent from the aluminum reduction plant, there were many particles of a diameter of less than 300 nanometers, but the capacity of the prior cascade impactor was not sufficient to capture and weigh a particle size less than 300 nanometers. The comments with respect to the particle size distribution in the effluent from the coal burning plant are applicable to the particle size distribution in the effluent from the aluminum reduction plant with respect to posing a health hazard and to posing a visibility hazard. Further, it is known that in the effluent from a Soderberg aluminum reduction plant that the effluent contains 3-, 4-benzopyrene which is a carcinogen and hazardous to the health of individuals.
In Seattle, Washington area, the particle size distribution in the atmosphere for Mar. 17, 18 and 19, 1966, was determined by capturing the particles by means of a thermal precipitator on a glass plate and, then by means of an electron microscope, determining the size of the particles captured. The size distribution ranged from 10 nanometers to 1000 nanometers. This is a typical particle size range for aerosols generated in the atmosphere.
The small particles in an aerosol may be the result of a comminution process whereby erosion reduces the size of a particle to form the smaller particle. An example is the grinding of metal, the rubbing together of solid material, the blowing of wind on rock, and many crushing and grinding operation that are common in industry. Another way of forming the small particles is the atmosphere is by a nucleation process whereby gases can condense or react to form tiny liquid or solid particles. After these particles have nucleated, they grow by coalescing with one another and/or by gas condensing on the particles to form larger particles. As a generalization, particles formed by the nucleation process are less than about 300 nanometers in diameter and particles formed by the comminution process are greater than about 300 nanometers in diameter.
There are means and methods for measuring particles having a diameter less than 1000 nanometers. One of these is the Aiken Counter which is capable of measuring the number of particles having a size less than 100 nanometers. The size of the particle itself is not measured by the Aiken Counter but the number of particles below 100 nanometers in diameter are measured. Also, there is a question in regard to the interpretation of the results of the Aiken Counter. This introduces a question of the reliability of the results of the Aiken Counter.
Another means is the diffusion cell for measuring the particle size. In the use of the diffusion cell, it is necessary to have an individual cell for each range of particle size. This means that the diffusion cell process is an expensive process and also a tedious process to use. With the diffusion cell, it is possible to measure a particle size in the range of about 10 nanometers in diameter.
Another means is the combination of a thermal precipitator and an electron microscope wherein the thermal precipitator captures all the particles and with the aid of the electron microscope the size of the captured particles can be determined. The means for capturing and determining the size of the particles is expensive and the process is a slow process.
Another means is a cascade impactor. In a cascade impactor, there are a number of stages, each stage comprised of a jet plate and a collection plate. As the aerosol-laden gas passes through the impactor, the gas is caused to pass through each jet plate and impinge on the corresponding collection plate. The gas velocity in each jet stage is higher than the velocity in the preceding stage. As the gas passes from stage to stage, each collection plate collects a smaller size range of particles than was collected by the preceding stage. The collection plates are weighed before and after the sampling period to determined the weight collected by each stage. In using a cascade impactor, the impactor is calibrated so that the particle size range captured on each plate is known. It is then possible, by means of the weight of the particles captured on the plate and the particle size range, to state the percent of particles by weight in a given stage or on a given plate. One of the disadvantages of the prior cascade impactors has been that a hard particle in an aerosol, such as fly ash from a coal burning plant, will bounce on the plate. A further disadvantage of a cascade impactor, prior to this invention, has been that a particle size less than 200 nm in diameter has not been captured except for research models operating at very low inlet pressure. A particle having a diameter less than 200 nm flows through the cascade impactor and is not captured for measurement and determination. In certain instances, particles having a size less than 200 nm have flowed through the cascade impactor and have been captured on a filter. The filter has been weighed so it is possible to know the aggregate weight of the particles having a diameter less than 200 nm but it has not been possible to determine the size of these particles.